The description relates to flat panel displays having overdrive functions.
An example of a flat panel display is a liquid crystal display including an array of pixels, each pixel having a liquid crystal layer that modulates the amount of light that passes the pixel. The brightness of a pixel can be controlled by applying a voltage having a particular value across the liquid crystal layer of the pixel to cause liquid crystal molecules in the layer to have particular orientations. To change a pixel from an initial gray level to a target gray level, the voltage applied to the pixel changes from a first value to a second value, causing the liquid crystal molecules to change from a first orientation to a second orientation. To reduce the amount of time required for changing the orientations of the liquid crystal molecules, an “overdrive” voltage can be applied to cause the liquid crystal molecules to move faster.
Because the viscosity of liquid crystals decreases as temperature increases, the response rate of a pixel (which depends on how fast the liquid crystal molecules can be re-oriented) increases as temperature increases. In some examples, the overdrive voltage is configured to be dependent on the temperature, such that less overdrive is used when the temperature is higher.
Referring to FIG. 1A, an example of a liquid crystal display 10 includes a non-volatile storage (e.g., a flash memory or an electrically erasable programmable read only memory (EEPROM)) 110, a thermal sensor 120, a timing controller 130, and a liquid crystal panel 140. The thermal sensor 120 senses the temperature of the liquid crystal panel 140. The non-volatile storage 110 stores lookup tables LUTa1˜LUTam, each storing an array of overdrive gray levels.
Each overdrive gray level represents a gray level that is sent to a data driver for over-driving a pixel from an initial gray level to a target gray level (the initial gray level may be the gray level of a pixel of a previous frame, and the target gray level may be the gray level of the pixel in the current frame). If the target gray level is higher (or lower) than the initial gray level, the overdrive gray level can be slightly higher (or lower) than the target gray level, so that the target gray level is reached faster.
Different lookup tables can be used for different temperatures. For example, the lookup table LUTa1 includes gray levels for overdriving the pixels during a temperature range T1 to T2. The lookup table LUTa2 includes gray levels for overdriving the pixels during a temperature range T2 to T3. When the display 10 is powered on, the timing controller 130 transfers the lookup table LUTa1 from the non-volatile storage 110 to a static random access memory (SRAM) that has a speed faster than the non-volatile storage 110, and uses the values in the lookup table LUTa1 for determining overdrive gray levels used to overdrive the pixels of the liquid crystal panel 140.
Referring to FIG. 1B, when the thermal sensor 120 determines that the temperature has risen to above T2, the timing controller 130 removes the lookup table LUTa1 from the SRAM. Referring to FIG. 1C, the timing controller 130 transfers the lookup table LUTa2 from the non-volatile storage 110 to the SRAM, then determines the overdrive gray levels based on the lookup table LUTa2. During a time period when the lookup table LUTa1 is being removed from the SRAM and the lookup table LUTa2 is being loaded into the SRAM, the timing controller 130 does not perform the overdrive function. For example, the amount of time required to remove and reload a lookup table having 40×40=1600 gray level values, each gray level value having 6 bits, is about 300 to 400 ms.
In some examples, the overdrive voltage is applied to the pixel circuit for a frame period (the time duration for displaying a frame). In some examples, a frame period is divided into two sub-frame periods. In the first sub-frame period, the overdrive voltage is applied to cause the liquid crystal molecules to quickly change to or near a desired orientation. In the second sub-frame period, a “normal” voltage is applied to maintain the liquid crystal molecules at the desired orientation, so that the pixel produces a desired gray level. Examples of overdrive techniques are described in U.S. Pat. No. 6,870,530, the contents of which are incorporated by reference.